1. Field of the Invention
This invention relates to a distance-measuring device, and more particularly to a distance-measuring device for use in the automatic focusing (AF) device of an active camera or video.
2. Description of the Related Art
There are two types of camera automatic focusing techniques: a passive system that makes use of luminance distribution information on the subject and an active system that projects a signal of infrared rays or the like onto the subject, and based on its reflected signal, determines the distance to the subject. Of them, the active system is more widely used for low-priced compact cameras because of its simple configuration.
FIG. 1 shows the construction of an infrared projection trigonometrical measurement system that projects infrared rays, senses the incident position of the reflected signal light, and determines the subject distance. In the figure, numeral 2 indicates an infrared light-emitting diode (IRED) acting as a light-projecting element, whose rays of light are gathered via a projection lens 4 onto the subject 6. The reflected signal light from the subject 6 is accepted by a reception lens 8, which directs it to a light-position sensing element (PSD) 10. The PSD 10 is an element that, if the position at which the light has arrived is x, produces two current signals I.sub.1 and I.sub.2 at each end according to the incident position.
Here, if the distance between the optical axis of the reflection lens 8 above the PSD 10 and one edge of the PSD 10 is a, equation (1) will hold: ##EQU1## where t is the length of the PSD 10, and I.sub.p is the total signal optical current that is expressed as: EQU I.sub.p =I.sub.1 +I.sub.2 ( 2)
If the distance between the optical axis of the projection lens and that of the reception lens is the base length S, and the distance between the reception lens 8 and the PSD 10 is f, the subject distance Z is expressed by equation (3): ##EQU2##
Thus, from equations (1) and (2), the following equations will hold: ##EQU3##
In this way, the subject distance l is obtained.
The AFIC 12 calculates the output signal currents I.sub.1 and I.sub.2 of the PSD 10 in the form of equation (4) at the same time that it actuates the driver 14 to causes the IRED 2 to emit light.
From equations (5) and (3), the following equation is obtained: ##EQU4##
AFDATA is defined as expression (7): ##EQU5##
From expression (7), the relationship between the reciprocal 1/l of the distance shown in FIG. 2 and AFDATA is determined.
Further, the CPU 16 computes 1/l based on equation (6), and controls the focusing lens. Specifically, 1/l is calculated from the following equation: ##EQU6##
In the aforesaid infrared projection trigonometrical measurement system, what is called a spot light deviation takes place. Specifically, when all of the projected signal light (the spot light 6a) is on the subject 6 as shown in FIG. 3A, the reflected signal light 3b correctly hits the PSD 10 as shown by the shaded portion in FIG. 3B. When the projected signal light (the spot light 6a ) is not completely on the subject and only half of the reflected signal light 6b' comes back, however, the position of the light point on the PSD 10 is shifted by .DELTA.x.
Since this system uses equation (3) as a basic equation, the deviation of .DELTA.x is converted to the distance .DELTA.l expressed by equation (9), leading to erroneous distance measuring: ##EQU7##
To remove this problem, for example, in Published Unexamined Japanese Patent Application No. 1-222235, two reception lenses 8a and 8b are placed symmetrically with the projection lens 4 as shown in FIG. 4. Specifically, with the system of the Published Unexamined Japanese Patent Application No. 1-222235, the deviation Ax on one PSD 10a has the opposite effect to that of Ax on the other PSD 10b. Therefore, taking an arithmetic means of the outputs of two PSDs 10a and 10b prevents erroneous distance measuring, thereby enabling the distance to be measured correctly.
This system, however, requires two reception lenses, resulting in a larger camera layout. Because the distance from one PSD to the AFIC becomes larger, the line is liable to be affected by noises. For this reason, the system is disadvantageous in terms of signal-to-noise (S/N) ratio.